Dart: Removes all but native calls and the handle watcher from the snapshot.

services/dart/test/pingpong_target/BUILD.gn

 # Copyright 2015 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 import("//mojo/public/dart/rules.gni")
 
 dart_packaged_application("pingpong_target") {
   output_name = "dart_pingpong_target"
   sources = [
     "main.dart",
   ]
   deps = [
+    "//mojo/public/dart",
+    "//mojo/public/interfaces/application",
     "//services/dart/test:pingpong_service_interface",
-    "//mojo/public/interfaces/application",
   ]
 }